This invention relates to the monitoring of thermal neutron flux within a nuclear reactor. More particularly, a new monitoring string having paired or grouped conventional local power range detectors and gamma thermometers is utilized, the disclosed utility including relating gamma thermometer output to a heat balance used for in-service life local power range detector calibration.
In the nuclear reaction interior of conventional boiling water reactors (BWR), it is possible to monitor the state of the reaction by either the measurement of thermal neutron flux or alternatively gamma ray flux.
Thermal neutron flux is the preferred measurement. As it is directly proportional to power and provides for a prompt (instantaneous) signal from a fission chamber. The alternative measurement of gamma radiation does not have the required prompt response necessary for reactor safety requirements. Consequently, gamma radiation as measured by gamma thermometers is not used to measure and immediately control the state of a reaction in boiling water nuclear reactors.
Boiling water reactors have their thermal neutron flux monitored by local power range detectors. These local power range detectors include a cathode having fissionable material coated thereon. The fissionable material is usually a mixture of U235 and U234. The U235 is to provide a signal proportional to neutron flux and the U234 to lengthen the life of the detector. The thermal neutrons interact with the U235 and cause fission fragments to ionize an inert gas environment, typically argon, interior of the conventional local power range detector. There results an electric charge flow between the anode and cathode with the resultant DC current. The amperage of the DC current indicates on a substantial real time basis the thermal neutron flux within the reactor core.
The boiling water reactor local power range detectors are inserted to the core of the reactor in strings. Each string extends vertically and typically has four spaced apart local power range detectors. Each detector is electrically connected for reading the thermal neutron flux in real time and for outputting the state of the reaction within the reactor. It is to be understood that a large reactor can have on the order of 30 to 50 such vertical strings with a total of about 120 to 200 local power range detectors. Such local power range detectors use finite amounts of U235 during their in-service life. Consequently the sensitivity changes with exposure. They must be periodically calibrated.
Calibration is presently accomplished by using traversing in-core probes or (TIPs). These traversing in-core probes are typically withdrawn from the reactor, as the traversing in-core probes are of the same basic construction as the local power range detectors and thus change their sensitivity with in-service life due to uranium 235 burnup.
In operation, the traversing in-core probes are typically calibrated. Such calibration includes inserting about five such probes separately to a common portion of a boiling water reactor. The boiling water reactor is operated at steady state and made the subject of an energy balance. The insertion of the traversing in-core probes occurs by placing the probes at an end of a semirigid cable and effecting the insertion within a tube system. Once a full core scan has occurred, during steady state operation, a heat balance is utilized in combination with the readings of the traversing in-core probes to calibrate the local power range detectors.
Thereafter, the newly calibrated traversing in-core probes travel through the reactor in a specially designed tube system. This tube system constitutes through containment conduits into the interior of the reactor vessel. Into these conduits are placed semirigid cables which cables have the TIPs on the distal end thereof. The TIPs are driven into the drive tube system from large drive mechanisms and the entire system is controlled from an electronic drive control unit. The cables pass through so-called "shear valves" which valves can shear the cable and seal the conduit to prevent through the tube system leaks, which leaks may well be substantial before the cable and probes could be withdrawn. The cables further pass through stop valves admitting the traversing in-core probes to the interior of the vessel containment. Finally, the cables reach so-called indexers, and then to the interior of the reactor vessel. These indexers are a mechanical system for routing each of the TIPs to pass adjacent the site of an assigned segment of the 170 some odd local power range detectors in a large boiling water nuclear reactor. It is normal for an indexer to include 10 alternative paths for a single traversing in-core probe to follow during a calibration procedure.
Needless to say, this system is elaborate and complex. Calibration of each local power range monitor is a function of the probe measurement of the local thermal neutron flux as well as a function of the position of the end of the inserting semirigid cable. Naturally, this position of the end of the semirigid cable has to be referenced to the proper alternative path for the necessary calibration to occur.
Further, the necessary tube system includes a matrix of tubes below the reactor vessel. Normally these tubes must be removed for required below vessel service and replaced thereafter.
Despite the presence of both stop valves and shear valves, the system remains as a possible escape route for water containing radioactive particles from the reactor. Further, the withdrawn cable can have mechanical complications as well as being radioactive.
Gamma thermometers are known. These thermometers measure the gamma ray output from a reactor reaction. Unfortunately, gamma ray output as measured by gamma thermometers does not provide a prompt response to power transients as required for safe operation of the reactor. Consequently, gamma thermometers have not been heretofore used for monitoring core reactive state in boiling water reactors.